Biodiesel, defined as fatty-acid alkyl ester (FAAE), or fatty acid methyl ester (FAME) is most commonly produced by a process of trans-esterification of triglycerides. The process involves reacting oils and fats with alcohol, usually methyl alcohol, in the presence of an alkaline catalyst. The conversion of triglycerides with alkaline catalysis is described in U.S. Pat. Nos. 2,383,601 and 2,494,366. The process is most efficient when the feedstock is a pure glyceride (refined oils and fats). The problem is that the price of the glycerides has increased dramatically over the last several years rendering it impossible to produce biodiesel that can compete with petrodiesel. Unfortunately, while cheaper feedstocks are available, they contain impurities including free fatty acids (FFA) that require additional processing, such as esterification or other, thus increasing the cost of producing biodiesel. The challenge is to develop processes that can allow production of cheaper feedstocks for producing biodiesel in one step.
The literature includes a number of approaches of dealing with FFA (see, Brown Grease Feedstocks for Biodiesel (2002) WWW domain nrel.gov, 2002, pp. 1-33, National Renewable Energy Laboratory, Boulder, Colo.). One of the options is to strip the FFAs from the oil. This is a well-known process, also known as physical refining or steam distillation. In this process, the FFA is stripped (evaporated) from the oil under vacuum. The FFA is condensed and recovered. The advantage of this process is that it produces oil that is practically free of FFAs and a very good feedstock for producing biodiesel. A challenge with this process is that there is a reduction in the amount of oil available to produce biodiesel due to loss of FFA and some neutral oil during the stripping process. Consequently, the higher the FFA the higher the yield loss and the lower the attractiveness of this approach. An example of this process of recovering fatty acids is set out in U.S. Pat. No. 6,423,857. This patent focuses on pre-treating high phospholipid containing oil (such as soybean oil) prior to steam distillation and subjecting the oil to steam distillation that produces a distillate containing at least about 97 percent by weight free fatty acids.
Another option is to react the FFAs with an alcohol, usually methyl alcohol, in the presence of an acid catalyst to produce FAAE/FAME. For instance, U.S. Pat. No. 4,164,506 discloses a biodiesel synthesis wherein fatty acids are subjected to acid catalysis. This process is called acid esterification and would be very attractive if it could convert all FFA into FAAEs. Unfortunately, this process poses several challenges: (a) un-reacted or unconverted FFA left in the oil after esterification must be removed with additional intermediate steps and equipment; (b) the esterification process requires use of acidic catalyst which poses risk to people (risk of burning skin and flesh upon contact) as well as equipment (risk of corrosion upon contact); and (c) the esterification process requires a large quantity of excess methanol (needed to maintain the proper equilibrium for advancing the reaction which is inhibited by the formation of water during esterification) thus increasing the emission of volatile substance in the atmosphere. The acid esterification is especially unattractive when the FFA content is higher because a large amount of acid catalyst and methyl alcohol are required in order to convert feedstocks having high FFA content. Water of reaction is also poses additional downstream processing problems as well as inhibiting esterification. Since the acid catalyst must be neutralized with alkali before processing the glycerides, the increased catalyst loading results in an excessive amount of salts produced as a consequence of alkali neutralization. Further, such processes generate a large volume of waste water as revealed in the disclosures of U.S. Pat. Nos. 4,303,590, 5,399,731 and 6,399,800.
Alternatively, solid catalysts can be used for the acid esterification reaction to avoid a neutralization step before the transesterification reaction. These processes have been extensively explored and documented, such as in U.S. Pat. No. 3,459,736 (which uses titanium oxide as a catalyst), U.S. Pat. No. 4,698,186 (which utilizes various solid catalysts), U.S. Pat. No. 4,267,393 which uses sulfonated resins as solid acid catalysts and U.S. Pat. No. 5,908,946 which employs zinc and aluminum oxide as catalysts for the esterification reaction).
U.S. Pat. Appl. No. 2003/0083514 discloses a single-phase process for production of fatty acid methyl esters from mixtures of triglycerides and fatty acids. This process is limited in that it requires acid catalyzed esterification of fatty acids prior to the transesterification step. U.S. Pat. No. 2,383,596 discloses a method for esterifying fatty acid and trans-esterifying glycerides. This process is limited in that only an esterification step is disclosed.
A third option is enzymatic catalysis. The conversion of both free fatty acids and triglycerides with enzyme catalysis is disclosed in U.S. Pat. Nos. 4,956,286, 5,697,986 and 5,713,965. A representative example of the esterification or transesterification method is disclosed in JP-B 6-65311, in which fatty acids or lower alcohol esters thereof are reacted with glycerol in the presence of an immobilized lipase having 1,3-position selectivity and the byproduct water or lower alcohol formed by the reaction is removed from the system at a reduced pressure to obtain the diglycerides. This reaction is preferably conducted in the presence of an enzyme having an ester activity, such as a lipase or an esterase, preferably in the presence of an immobilized or intracellular lipase having 1,3-position selectivity. Known methods for immobilization are described, for example, in “Koteika Koso (Immobilized Enzyme),” edited by Ichiro Chihata, published by Kodansha Ltd. Publishers, pp. 9-85 and “Koteika Seitai-shokubai (Immobilized Biocatalyst)” edited by Ichiro Chihata, published by Kodansha Ltd. Publishers, pp 12-101. Immobilization onto an ion-exchange resin is preferred. Lipases having 1,3-position selectivity and usable in immobilization include those derived from microorganisms of, for example, the genera Rhizopus, Aspergillus, Mucor, etc., as well as pancreatic lipases, and the like. For example, use can be made of the lipases derived from Rhizopus delemar, Rhizopus japonicus, Rhizopus niveus, Aspergillus niger, Mucor javanicus, and Mucor miehei. A commercial immobilized lipase having 1,3-position selectivity is Lipozyme® IM, manufactured by Novo-Nordisk Bioindustry A.S. An intracellular lipase having 1,3-position selectivity comprises a lipase having 1,3-position selectivity adsorbed or bonded to microbial cells. A commercially available example thereof is Olipase™, manufactured by Nagase & Co., Ltd. This process is challenging because the reaction produces water which inhibits the forward reaction. Other problems with enzymatic processing are the slow reaction rates and high cost of enzymatic catalysts. Further, enzymatic catalysts have a limited life. These shortcomings when compared to alkaline and acidic reactions render the enzymatic processes economically unfavorable.
A fourth option is described in US Pat. Appl. No. 2012/0123140 involving glycerolysis of high free fatty acid (HFFA) oil. This process converts FFAs into oils through esterification of fatty acids with glycerol. The resulting product is oils which are fatty acid glycerin esters (or FAGE). This process is variously known as glycerolysis, alcoholysis, or esterification. Glycerolysis of fats and oils with glycerol has been intensively researched during the 1940's and 1950's. Sonntag (1982) (Sonntag, N. o. V., glycerolysis of Fats and Methyl Esters—Status, Review, and Critique, Journal of American Oil Chemists Society 59:795A-802A) has a complete collection of these patents in his review. The reaction produces a mixture of mono-, di- and tri-glycerides.
For example, U.S. Pat. No. 3,102,129 discloses a process for producing monoglycerides of fatty acids and U.S. Pat. No. 2,875,221 discloses a process for preparing monoglycerides of fatty acids. These processes are limited in that they require admixing a substantial proportion of previously reacted monoglyceride product with a freshly mixed stream of glycerol and fat and rapidly heating the mixture on a hot surface. U.S. Pat. No. 6,500,974 discloses a process for preparation of a monoglyceride. This process is limited in that the presence of a food grade polar solvent is required in the glycerolysis reactor.
Although the esterification or transesterification method is a process in which fatty acids or lower alcohol esters thereof and glycerol are converted to partial diglycerides through a one-step reaction, it is not cost efficient because the individual feedstock materials are expensive. For conducting the second stage esterification reaction, glycerol is added to the partial decomposition product, obtained through the first-stage reaction in such an amount that the mole number of fatty acid groups in the decomposition product mixture of the first stage is from 0.8 to 2.5 mol per 1 mol of glycerol groups based on the total of glycerol groups of the decomposition product mixture of the first stage and glycerol groups added to the second stage (see, e.g., U.S. Pat. No. 6,261,812).
On the other hand, U.S. Pat. No. 2,808,421 discloses a method for preparing mixed triglyceride compositions using a titanium alcoholate catalyst. U.S. Pat. Nos. 7,806,945, 8,088,183, 7,871,448, and US. Pat. Appl. No. 2012/0123140, disclose a process for preparation of fatty acid methyl ester using HFFA oil. The process includes glycerolysis as part of their overall process. The conditions taught for glycerolysis of free fatty acids (at a temperature of about 220.degree. C and at a pressure of about 2 pounds per square inch absolute) in a glycerolysis reaction without a catalyst to produce a glycerolysis reactor effluent stream that contains less than 0.5 percent by weight of free fatty acids and a plurality of glycerides, are similar to other literature. These patents teach there is a need for at least two continuous stirred tank reactors that are operated in series with a combined residence time of not more than about 500 minutes. For a 20% FFA stream, the time taken is no more than 200 minutes. A problem with this approach is that, despite claims to the contrary, it only efficiently reduces the FFA by 80-90%, thus making it necessary to either use catalysts or add intermediate steps and equipment to reduce the remaining FFA either chemically or physically. Moreover, the size of glycerolysis reactors is large because it is sized to handle the entire mass of oil even though the FFA content is a relatively small portion of that stream and consequently there is a waste of energy because a greater amount of material (the entire HFFA oil stream) is subject to higher temperature and then cooled down when it is only necessary to heat the FFA.
The background art is also characterized by a number of non-patent publications. Noureddini et al. in glycerolysis of Fats and Methyl Esters, JAOCS, 1997, pp. 419-425, vol. 74, no. 4 discloses the glycerolysis of methyl esters and triglycerides with crude glycerin. The main focus of their study is on utilization of “crude” glycerol obtained from the biodiesel industry as opposed to “pure” glycerin previously used in glycerolysis to mono-, di-, and tri-glycerides. They did not disclose glycerolysis of fatty acids and their focus was on production of mono- and di-glycerides from FAME and tri-glycerides using crude glycerin.
Felizardo, et al. in “Study on the glycerolysis reaction of High Free Fatty Acid Oils for Use as Biodiesel Feedstock”, Fuel Processing Technology, 2011, pp 1225-1229, vol. 92, no. 6, discloses the conversion of oils with a high content of FFA (20-50%) by esterification with glycerol. The results suggest that the FFA content could be reduced from 50% to 5% in 3 hours at 200 degree. C. without the use of a catalyst. The presence of a zinc-based catalyst reduced the reaction time to 1 hour and reduced the FFA to 1.2%.
Canakci, M. and J. Van Gerpen (2001) in Biodiesel Production from Oils and Fats with High Free Fatty Acids, Transactions of the American Society of Agricultural Engineers, 44(6):1429-1436 discloses that “glycerolysis” is an alternative process that can be used with feedstocks containing more than 10% FFAs. This involves adding glycerin at 400.degree. F. and letting it react with the FFAs to form monoglycerides, a glycerol molecule to which one free fatty acid has been joined. These monoglycerides can then be processed using a standard alkaline catalyst transesterification process. Waste glycerin from biodiesel processing can be used in this process. Glycerolysis can be expensive because of the high heat involved, which requires a high-pressure boiler and trained boiler operator. Also, a vacuum must be applied while heating to remove water that is formed during the reaction. Another disadvantage is that the glycerin will also react with the triglycerides in the oil to convert some of them to monoglycerides. While this does not negatively impact the reaction, it means that more glycerin is required for the process, and therefore more glycerin must be removed at the end of the transesterification.
Kumoro in “Experimental and Modeling Studies of the Reaction Kinetics of Alkaline-Catalyzed Used Frying Oil Glycerolysis using Isopropyl Alcohol as a Reaction Solvent, Research Journal of Applied Sciences, Engineering and Technology 4(8): 869-876, 2012, discloses a glycerolysis process using isopropyl alcohol and an alkaline catalyst. However, the focus of this and several other researches is to convert tri-glycerides to mono-glycerides for use in foods, cosmetics, and pharmaceutical products. This study is not directly relevant to our invention because it does not address glycerolysis of fatty acids.
Tyson in Brown Grease Feedstocks for Biodiesel, WWW domain nrel.gov, 2002, pp. 1-33, National Renewable Energy Laboratory, Boulder, Colo., discloses techniques for converting greases to biodiesel. The techniques disclosed in this reference are limited. Moreover, the conditions taught for glycerolysis of free fatty acids are at temperatures in the range of 250.degree. C. to 260.degree. C. in the absence of a catalyst or at 220.degree. C. with a catalyst. The reference teaches that there is “no proven technology for 50+% FFA mixes” and that “combined processes for ASTM [American Standard for Testing and Materials] quality biodiesel not well developed, technical and economic questions exist.”
Tyson in Biodiesel Technology and Feedstocks, WWW domain nrel.gov, 2003, pp. 1-37, National Renewable Energy Laboratory, Boulder, Colo., includes much of the same information as contained in her 2002 presentation. The reference notes that using “glycerolysis to treat FFA” to “convert FFA to monoglycerides, then transesterify” is “commercial, not currently used in biodiesel.”
Davis Clements in Pretreatment of High Free Fatty Acid Feedstocks, Biodiesel Production Technology Workshop III, Mar. 26-28, 2003, pp. 78c-78i, University of Nebraska, Lincoln, Nebr., discloses a number of methods for pretreatment of high free fatty acid feedstocks prior to transesterification. This process is limited in that glycerolysis is carried out at 200.degree.C. under an 11 pounds per square inch vacuum, usually with a catalyst such as zinc chloride, with venting of water. This process is further limited in that, in the absence of a catalyst, a residence time of over 5 hours is required to achieve an effluent containing less than 1 percent free fatty acids.